Coated abrasive grains encapsulated in a cross-linked thermoset polymmer

ABSTRACT

THE MANUFACTURE OF ABRASIVE ARTICLES IS FACILITATED BY ENCAPSULATING ABRASIVE GRAINS WHICH ARE COATED WITH A BINDER I A THIN SHELL OF A CURED POLYISOCYANATE/HYDROXYL FUNCTIONAL POLYMER SYSTEM. THE ENCAPSULATED ABRASIVE GRAINS ARE DRY AND TACK-FREE AND MAY BE STORED AND PROCESSED IN THE TACK-FREE FORM INCLUDING PROCESSING THROUGH A PRESSING OPERATION TO FORM GREEN ABRASIVE ARTICLES SUITABLE FOR BAKING OR FIRING.

United States Patent Olhce 3,806,327 Patented Apr. 23, 1974 3,806,327 COATED ABRASIVE GRAINS ENCAPSULATED IN A CROSS-LINKED THERMOSET POLYMER Donald A. Farmer, North Haven, Conn., and John D. Kennedy, Mahopac, N.Y., assignors to Ashland Oil Inc Houston, Tex.

No Drawing. Filed June 14, 1971, Ser. No. 158,133 Int. Cl. B24d 5/02; C08g 51/12 US. Cl. 51--295 10 Claims ABSTRACT OF THE DISCLOSURE The manufacture of abrasive articles is facilitated by encapsulating abrasive grains which are coated with a binder in a thin shell of a cured polyisocyanate/hydroxyl functional polymer system. The encapsulated abrasive grains are dry and tack-free and may be stored and processed in the tack-free form including processing through a pressing operation to form green abrasive articles suitable for baking or firing.

DISCLOSURE This invention relates to abrasive articles and improved processes for the manufacture of abrasive articles. More particularly this invention provides for an improved tackfree adhesive-coated abrasive grain or abrasive mix which can be stored or completely processed and pressed in the tack-free condition to form abrasive articles suitable for baking or firing.

BACKGROUND OF THE INVENTION In this disclosure the term abrasive articles refers to those products which are essentially abrasive aggregates bonded together or to a substrate with various types of binders. Among these products are cutoff wheels, snagging wheels, reinforced wheels, and other grinding wheels.

Such wheels are manufactured in various configurations, e.g., disks, cones, cylinders, segments, cups and sticks. Other such products may include abrasive belts or plates wherein the abrasive aggregate is bonded in various thicknesses to a flexible or rigid substrate. The above abrasive articles are usually made by coating an abrasive aggregate or abrasive grain with hinder or bond, a mixture of binder and filler material, to form an abrasive mix, e.g., a resin coated abrasive aggregate. This abrasive mix is then cold pressed or hot pressed under high pressure, e.g., 1500 to 5000 p-.s.i., to form a green abrasive article. The green strength of the green abrasive article is believed to be normally due partially to the mechanical forcing of the hinder or bond coating on each abrasive grain to fuse with the binder or bond coating on other abrasive grains and (especially in hot pressing) partially due to a slight curing of the binder in the abrasive mix. Green strength must be adequate to provide accurate dimensional stability when the green abrasive article is released from the press mold. The green abrasive article is then baked or fired at relatively high temperatures over periods of times sufficient to fully cure or set the binder. Of course, some abrasive articles are made by baking or firing the green abrasive article while it is in the press mold itself, thus requiring little or no green strength. The adhesive or binder employed is usually a resinous or polymeric material in the case of resinoid abrasive articles and usually a powdered ceramic in the case of vitrified abrasive articles. The density of the abrasive article is normally a function of the pressure used to form the green abrasive article, as well as the average size, shape and size distribution of the abrasive grain and the amount and type of hinder or bond employed.

As seen from US. Pats. 2,901,337; 2,943,926; and

3,208,836, abrasive articles are conventionally made with a powdered binder such as a novolak phenolic resin. Resole phenolic resins are not generally used alone due to the fact that they are usually in a liquid or semi-liquid form. The powdered resin has been used in order to obtain the free-flowing abrasive mix which is sufliciently tack-free for use in automatic pressing equipment.

Since a high percentage of abrasive articles are made in high speed, automatic equipment, the abrasive'mix must be able to flow intermittently through the conduits of such equipment without agglomerating when the flow is stopped. In some cases it has been necessary to dust the abrasive mix with a dry inert powder, such as silicon dioxide, to impart the necessary degree of flowability. However, if the abrasive mix was too dry, sufiicient green strength would not be achieved because the adhesive coating on the abrasive grains will not fuse adequately when pressed. In these cases it has been necessary to add a tackifier, such as furfural, to the abrasive mix to obtain the desired green strength. See US. Pat. 3,208,836. This, of course, increased the chances of the mix agglomerating in the processing equipment.

When a dry powdered binder is used in accordance with conventional procedures some means must be used to get the dry powdered binder to adhere to the dry abrasive grain. This has been done by first coating or wetting the abrasive grains with a small amount of liquid resin such as a one-step phenolic resin, known as a resole. The resole wets the surface of the abrasive grains and causes the dry powdered binder to adhere to the abrasive grains. The use of the resole could be avoided as described in US. Pat. 2,943,926, wherein an adhesion-promoting agent, such as cresol, was used to cause the powdered binder to adhere to the abrasive grains. Other wetting agents which are solvents for the adhesive or binder have also been used. In order to obtain sufficient wetting of the abrasive grain surface it was sometimes necessary to use a resole having a high volatile content, which would then produce blistering and warping of the adhesive article during the baking or firing step.

When the novolak resin is employed it is necessary to add a source of formaldehyde, such as hexamethylenetetramine (known as hexa), to allow complete curing during the baking step. If a resole could be used, the additional source of formaldehyde would be unnecessary, because the resole contains sufficient formaldehyde for complete curing during baking. According to the prior art, the resole resin is not normally used alone because of the difiicult'y in obtaining free-flowing abrasive mix.

As noted above, the addition of a tackifier, such as furfuryl alcohol, to the abrasive mix is sometimes necessary to obtain the necessary green strength. The use of a tackifier usually transforms a dry abrasive mix into a tacky mass, which has a greater tendency to agglomerate in the processing equipment conduits. Thus, the conventional practice has necessitated operating a portion of the processing and pressing equipment with a tacky abrasive mix contained therein. Experience has shown that a delay in processing (e.g., caused by mechanical malfunction), or

an incorrect control of operating temperature, humidity, or flow rate can cause this tacky abrasive mix to agglomerate in the holding or feed portions of the processing equipment. When the abrasive mix gells in the equipment, it causes considerable loss of time and equipment efficiency due to the time necessary to remove the gelled mass from the equipment. In some instances conduits must be discarded and replaced because it is impossible or impractical to remove the gelled abrasive mix from them.

In summary, a conventional abrasive mix which will give satisfactory green strength is not suitable for storage, because it is too tacky and will agglomerate upon standl 3 7 ing. Conversely, a conventional abrasive mix which is sufiiciently tack-free for storage will not give the required green strength when pressed. As a result, conventional abrasive mixes are inconvenient to handle in that a mix suitable for storage must be treated with a tackifier before use and if it is not then used must be discarded or dusted again before storage.

THE INVENTION In general, this invention involves forming an abrasive mix by coating an abrasive grain with a binder or bond composition then encapsulating the coated abrasive grain in a thin shell of cross-linked thermoset polymer encapsulant system. Thermoset as used herein means a material which will not become fluid upon heating and cross-linked means a material with an infinite molecular weight. It is important in the present invention that a catalytic agent be present in the binder or bond coating on the abrasive grain in order to cure or cross-link the encapsulant system.

This invention is not only useful with conventional binders for abrasive aggregates, as described above, but provides methods of using binders which have not heretofore normally been used in the manufacture of abrasive articles. Thus, this invention can be employed to encapsulate a conventional abrasive mix to eliminate the necessity of dusting the mix to make it storable and eliminate the necessity of adding the tackifier before using the mix. Alternatively, this invention allows the use of semi-liquid binders which will adhere to a dry abrasive grain thus eliminating the necessity of first coating the abrasive aggregate with a wetting agent then coating the aggregate with a powdered binder.

The present invention eliminates not only the problem of agglomeration in the processing equipment but pro vides additional conveniences, such as allowing a press machine and the associated feed equipment to be stopped for extended lengths of time without removal of the abrasive mix from the machinery. Obviously, this allows for increased machine running time since cleaning and loading operations at daily shut-down and start-up are eliminated. Of course, further efliciency is achieved due to the fact that the operation of adding and mixing the tackifier is also eliminated.

This invention provides a free-flowing, tack-free abrasive mix which is storable and usable without additional or special treatment for either. Conventional abrasive mixes have a tendency even when dusted with conventional dusting powders to agglomerate during storage, particularly when stored in humid conditions. In some cases an additional storage problem is encountered in that the wetting agent holding the binder or bond to the abrasive grain tends to evaporate allowing the binder or bond to separate from the abrasive grain. The present invention overcomes these storage problems by providing an abrasive m'ur having resistance to attack by humidity and providing coated abrasive grains encapsulated in a thin shell of cross-linked thermoset polymer which prevents evaporation of the wetting agent and prevents separation of the hinder or bond from the abrasive grain.

In one particular embodiment of this invention the abrasive grains and a semi-liquid resole phenolic resin having a low volatile content are mixed until the grains are uniformly coated with the resin. A catalytic material, such as a basic material is added to the abrasive mix and the mix is agitated while an encapsulant system, such as a. polyisocyanate/benzylic ether phenolic resin mixture, is applied to the grains. The encapsulant system instantly cures upon contact with the catalytic material thus forming a thin, fully-cured shell encapsulating each abrasive grain. The abrasive grains are then completely non-tacky, free-flowing and may be stored, processed and ultimately used without further treatment.

As mentioned above, one particular advantage of this invention manifests itself in the fact that the encapsulated 4 1 grains per se can then be completely processed through standard equipment and cold pressed to form green abrasive articles having good green strength. While not wishing to be bound by theory, it is believed that this is made possible by the pressure applied during the pressing step breaking each of the encapsulating shells, thus exposing the adhesive on each abrasive grain and allow ing it to bind the grains together. Moreover, it is believed that the cross-linked encapsulant system constitut ing the thin shell may further react or cross-link with the adhesive and become part of the overall binding adhesive mass in the final abrasive article. Thus, the final abrasive article may have more actual binding material or adhesive in it and less impurities and fillers than heretofore possible.

In another embodiment of this invention, the above step of adding the catalytic agent can be eliminated by using an adhesive that already contains a catalytic agent. For example, when using a polyisocyanate/benzylic ether phenolic resin encapsulant system, the hexa conventionally present in a novolak binder serves as the catalytic agent.

The abrasive aggregates useful in forming abrasive articles according to this invention include alumina, silicon carbide, diamond, zirconia, garnet, emery, or other conventional aggregates. The particle size of the abrasive grain may vary over a wide range, such as from 6 to 1200 grit, but generally abrasive grain in the range of 8 to grit is used in grinding wheel applications.

The adhesive or hinder useful in this invention may be any conventional resinous or ceramic binder, such as phenolic resins or feldspars, respectively. Suitable phe nolic resins may be produced by the alkali catalyzed con densation of excess formaldehyde, i.e., a resole, or produced by the acidic catalyzed condensation of excess phenol with formaldehyde, i.e., a novolak. The novolak resin is generally in the form of a dry powder which necessitates the use of a wetting agent on the abrasive grain so the novolak will adhere to the abrasive grain and generally requires the use of an additional formaldehyde source, such as hexa, which releases additional formaldehyde for further curing of the novolak when the abrasive article is baked. The resole resins are generally liquid or semi-liquid but may be powdered and for purposes of this invention should have a volatile content below about 30% by weight based on the total weight of the resin and preferably below about 20%. lf the volatile content of the resin is too high, the abrasive article may develop blisters or may warp during the baking operation. Depending on the porosity of the abrasive article the baking or firing conditions may usually be adjusted to allow suflicient time for the volatile materials to escape, thus avoiding blistering or warping. Other resinous binders useful in this invention include alkyl resins, furans, polyamides, shellacs, epoxies, rubbers, or mixtures thereof. Other ceramic binders include kaolins, china clays, ball clays, and commercially available fritted bonds. In general the ceramic binders are mixed with water and some organic polymer, e.g., dextrin, urea resin, etc, for purpose of forming an abrasive mix which will give adequate green strength.

Encapsulant system as used in this disclosure refers to a liquid system containing a polyisocyauate and a hydroxyl functional polymer selected from the group consisting of water-free phenolic resins, polyester resins and epoxy resins. The encapsulant system or either of the components may contain solvents.

The polyisocyanates useful in the encapsulant system of this invention are aliphatic, cycloalip'hatic, or aromatic polyisocyanates having 2 or more isocyanate groups. If desired, mixtures of polyisocyanates can be employed. Less preferably, isocyanate prepolymers formed by re acting excess polyisocyanate with a polyhydric'alcohol, e.g., a prepolymer of toluene diisocyanate and ethylene glycol, can be employed. Suitable polyisocyanates include the aliphatic polyisocyanates such as hexamethylene diisocyanate, alicyclic polyisocyanates such as 4,4'-dicyclohexylmethane diisocyanate, and aromatic polyisocyanates such as 2,4'- and 2,6-toluene diisocyanate, diphenylmethyl diisocyanate, and the dimethyl derivatives thereof. Further examples of suitable polyisocyanates are 1,5-naphthalene diisocyanate, triphenylmethane triisocyanate, xylylene diisocyanate, and the methyl derivatives thereof, polymethylenepolyphenol isocyanates, chlorophenylene-2,4-diisocyanate, and the like. Although all polyisocyanates react with hydroxyl functional polymers to form a cross-linked polymer structure, the preferred polyisocyanates in the case of water-free phenolic resins are aromatic polyisocyanates and particularly diphenylmethane diisocyanate, triphenylmethane triisocyanate, and mixtures thereof.

The polyisocyanate is employed in suflicient amounts to cause the curing of the hydroxyl functional polymers. In general, the polyisocyanate will be employed in a range of to 500 weight percent of polyisocyanate based on the weight of the hydroxyl functional polymer. Preferably, from 20 to 300 weight percent of polyisocyanate on the same basis is employed. The polyisocyanate is employed in liquid form. Liquid polyisocyanates can be employed in undiluted form. Solid or viscous polyisocyanates are employed in the form of organic solvent solutions.

The water-free phenolic resins useful in the encapsulant system include benzylic ether phenolic resins, novolaks, resoles, and substituted phenolic resins.

The benzylic ether phenolic resin useful in this invention is prepared by the reaction of a phenol havin the formula:

wherein X, Y and Z are each hydrogen, hydrocarbon radicals, oxyhydrocarbon radicals, or halogen with an aldehyde having the formula RCHO wherein R is a hydrogen or hydrocarbon radical of 1 to 8 carbon atoms. The most preferred aldehyde is formaldehyde. The mole ratio of aldehyde to phenol is greater than 1; the reaction is conducted in the liquid phase which is substantially anhydrous at a temperature below about 130 C. in the presence of catalytic concentrations of a soluble metal salt dissolved in the reaction medium. Examples of suitable salts are the naphthenates, neodecanoates, octoates and lactates of lead, calcium, zinc, tin, manganese, copper and magnesium. The benzylic ether phenolic resin ordinarily has the general formula:

1 CH A wherein R is hydrogen, hydrocarbon radical, oxyhydrocarbon radical or halogen, m and n are numbers the sum of which is at least 2, A is hydrogen or methylol end group and wherein m is at least 1 and the sum of m and the number of methylol end groups is at least 2. The benzylic ether phenolic resin is the preferred hydroxyl functional polymer.

The novolak resin useful in the encapsulant system of this invention is prepared by the reaction of a phenol having the formula:

wherein X, Y and Z are each .hydrogen, hydrocarbon radicals, or halogen and wherein at least two Ys are hydrogen with an aldehyde having the formula ROHO wherein R is a hydrogen or hydrocarbon radical of 1 to 8 carbon .atoms. The most preferred aldehyde is formaldehyde. The mole ratio of aldehyde to phenol is less than 1; the reaction is conducted in aqueous phase at a temperature below about C. in the presence of catalytic concentrations of an acid, such as sulfuric, hydrochloric, oxalic, salicylic, acetic, trichloroacetic or dichloroacetic acids. Certain Lewis acids may be useful, such as borontrifluoride.

The resole resin useful in the encapsulant system of this invention is prepared the same as the above benzylic ether phenolic resin except the reaction of the phenol and aldehyde is conducted in aqueous phase and under an alkaline pH.

The polyesters useful in the encapsulant system are those which are essentially neutral and contain a substantial percentage of phenolic hydroxyl groups. Examples of such polyesters are those prepared by co-condensing (1) a phenolic resin and a polyester resin, (2) a polyester and a phenol substituted with functional group reactive with polyesters, such as methylol, ether or acetyl groups, and (3) a dibasic acid or anhydride and a phenol substituted with functional groups: reactive with the acid or anhydride. In general, such polyesters should contain a ratio of phenolic hydroxyl groups to aliphatic hydroxyl groups of at least 3 to l. The polyesters may be pre pared by conventional condensation techniques using the necessary proportions to give the above ratio of phenolic and aliphatic hydroxyl groups.

The epoxy resins useful in the encapsulant system are those which contain a substantial proportion of phenolic hydroxyl groups. Examples of such epoxy resins are those prepared by co-condensing 1) a phenolic resin and an epoxy resin or (2) a phenolic resin with an epoxy monomer. The ratio of phenolic hydroxyl groups to aliphatic hydroxyl groups should be at least 3:1. These epoxy resins may be prepared by conventional condensation techniques.

In addition, the above epoxy resins or polyester resins may be mixed with the above water-free phenolic resins for use in the encapsulant system.

As mentioned above, the adhesive in the abrasive mix must contain a material which acts as a catalytic agent to cross-link the encapsulant system when applied to the abrasive mix. While not wishing to be bound by theory, it is believed that the catalytic agent causes the encapsulant system to cure or cross-link instantly upon contact with the surface of the coated abrasive grain thus forming a rigid film on the surface of that coated grain.

The catalytic agent useful in this invention is a base, metal ion, or amine. The catalytic agent should be evenly distributed in the abrasive mix to give rapid curing of the encapsulant system upon contact with the surface of a coated abrasive grain. The catalytic agent may be basic materials which retain their basic characteristics or become basic when added to or mixed with the adhesive or the abrasive mix. Such basic materials include such compounds as lime, sodium hydroxide, lithium hydroxide, potassium hydroxide and barium hydroxide. The metal ion catalytic agent may be a mono'valent, divalent or trivalent metal ion. Although others may be used, the preferred metal ions include lead, calcium, zinc, tin, manganese, copper and magnesium. The metal ion may conveniently be employed as a metal salt, i.e., a compound wherein the metal is ionically bonded to the salt radical. Examples of suitable salts are the neodecanoates, naphthenates, octoates, lactates and dilaurates of the abovementioned metals. The amine catalytic agents useful in this invention include liquid and solid tertiary amines but normally gaseous tertiary amines may be employed if dissolved in a liquid solvent. Examples of suitable amines are trimethyl amine, triethylamine, hexamethylenetetramine, triethylenetetramine, tributylamine and those tertiary amines derived from diethylenetriamine. The catalytic agent may be present in the abrasive mix as part of the binder or bond, for example the hexamethylenetetramine in a novolak binder will serve as the catalytic agent, or may be added separately to the abrasive mix, for example the abrasive mix may be lightly dusted with lime or other solid catalytic agent.

Although the solvent employed in combination with either the hydroxyl functional polymer or the polyisocyanate or in the encapsulant system as a whole does not enter to any significant degree into the reaction between the polyisocyanates and hydroxyl functional polymer in the presence of the catalytic agent, it can affect the reaction. Thus the difference in the polarity between the polyisocyanate and the hydroxyl functional polymer restricts the choice of solvents in which both components are compatible. Such compatibility is necessary to achieve complete reaction. For example, polar solvents of either the protic or aprotic type are good solvents for the benzylic ether phenolic resin, but have limited compatibility with the polyisocyanates. Aromatic solvents, although compatible with the polyisocyanates, are less compatible with the benzylic ether phenolic resins. It is therefore preferred to employ combinations of solvents and particularly combinations of aromatic and polar solvents. Suitable aromatic solvents are benzene, toluene, xylene, ethylbenzene, and mixtures thereof. Preferred aromatic solvents are mixed solvents that have an aromatic content of at least 90% and a boiling point range within a range of 280 to 450 F. The polar solvents should not be extremely polar such as to become incompatible with the aromatic solvent. Suitable polar solvents are generally those which have been classified in the art as coupling solvents and include furfural, furfuryl alcohol. Cellosolve acetate, butyl Cellosolve, butyl Carbitol, diacetone alcohol, and Texanol.

The amount of the solvent present in the encapsulant system generally will not exceed about 60% by weight based on the Weight of the encapsulant system. For example, when the benzylic ether phenolic resin is used the preferred solvent level is about 45% by weight of the encapsulant system.

In normal practice of this invention the abrasive aggregate may be mixed with up to about 50% by weight of total bond material based on the weight of aggregate, wherein the bond includes both binder and fillers. Generally the bond material is present in an amount between about and 35%. For grinding wheels and cutoff wheels the preferred level of the bond material is between about 10% and 25%. The bond material generally may be from 100% to about 40% binder by weight. The actual bond level and ratio of binder to filler employed in any particular abrasive article will depend on the use intended for the article, the type of aggregate, the density desired and the type of binder used as will be apparent to one skilled in the art. Inert or functional filler materials may be used in the bond. Functional filler materials are those which promote grinding efficiency.

' The amount of encapsulant system employed is generally between about 0.5% and 4% by weight based on the weight of the abrasive mix. While the optimum amount will vary depending on the type and amount of bond in the abrasive mix, the size of the abrasive grain and the total surface area to be coated, the preferred amount 8 is between about 0.5% and about 2.0%. For grinding wheel and cutoff wheel applications the preferred amount of encapsulant system is between about 0.5% and about 1.5% by weight based on the weight of the abrasive mix.

The encapsulant system may be incorporated in the abrasive mix in any convenient manner, such as spraying or dripping. The abrasive mix must be agitated, tumbled or otherwise placed in motion while the encapsulant system is added to avoid encapsulating agglomerated abrasive grains and forming aggloinerates of abrasive grains. Room temperature is usually employed, but is not necessary.

The catalytic agent should be present in the abrasive mix in an amount of at least equal to about 0.1% by Weight based on the weight of the abrasive mix. Much higher concentrations of catalytic material may be present, for example when hexa is present with a novolak binder. When the binder does not contain a basic material, a metal ion or teritary amine or does not contain sufficient amounts of these materials, it will be necessary to add at least one such material to the binder or to the abrasive mix to provide the above mentioned levelof catalytic material in the abrasive mix. When a dry catalytic material, e.g., lime, is added to the surface of the coated grains in the abrasive mix, a lower concentration of catalytic material in the abrasive mix will usually be sufiicient than when the catalytic material is premixed with the binder before the abrasive mix is formed.

To form the green abrasive articles the encapsulated abrasive mix is hot or cold pressed in a mold under at least about 500 p.s.i. Normally pressures between about 1000 psi. and about 6000 p.s.i. are employed. For grinding wheels and cutoff wheels pressures between about 2000 psi. and about 4000 psi. are preferred.

The green abrasive article is then baked or fired at conventional temperatures, which range from about 280 F. to about 375 F. for baking and range from about 1800 F. to about 2400 for firing, and for conventional periods of time.

The green abrasive article is then baked or fired at conventional temperatures, which range from about 280 F. to about 375 F. for baking and range from about 1800 F. to about 2400 for firing, and for conventional periods of time.

The practice of this invention produces superior abrasive products. For example, grinding wheels pressed from the free-flowing encapsulated abrasive mix of this invention have fewer flaws and more even strength characteristics than conventionally formed wheels. It is believed this is due to the fact that the free-flowing encapsulated abrasive mix of this invention gives a more consistant density throughout the wheel than will a conventional tacky abrasive mix. Thus, during pressing there are fewer internal stresses developed. Another contributing factor is believed to be the fact that the mix may be wetter when the powdered binder is distributed thereon before encapsulation, a more uniform distribution of binder is achieved. The encapsulant system then rigidly maintains this uniform distribution throughout the manufacturing process. It is believed that the combination of the more consistent density and more uniform distribution of the binder account to a large extent for the improved properties of wheels or other articles made according to this invention. Of course, the more even density provides wheels which are easier to balance and which remain in balance better while being used.

In order to further point out the full nature of this invention the following examples illustrate embodiments thereof and do not illustrate the scope thereof. In the following examples the abrasive mix is formed by wetting the abrasive grain, coating the wetted grain with the bond which is a mixture of the binder and filler. -A portion of the abrasive mix is used as the control and another portion is encapsulated according to this invention. A set of wheels are made from each portion of the abrasive mix and sub- EXAMPLE 1 Aggregate mix: Parts by weight Aluminum oxide (30 grit) 79 Furfural Alcohol wetting agent 2 Novolak resin (with 9.0% by weight hexa) 11 Cryolite 8 Encapsulant system:

Benzylic ether phenolic resin 50 Polyisocyanate 50 The novolak resin is prepared by heating 73 parts by weight of 37% aqueous solution of formaldehyde, 100 parts phenol and 0.2 parts H 80 to boiling and refluxing for 15 minutes. The pH is adjusted to 1.2 or below and refluxing continued for an additional hour and 45 minutes. Ammonia is added and boil-off conducted at 150 C. for steam distillation to a melting point of about 235 -F. The mixture is then dumped in a pan and allowed to cool and solidify. The solid is then ground with the hexa to a powder.

The benzylic ether phenolic resin for the encapsulant system is prepared by charging 1014 g. of phenol, 720 g. of paraformaldehyde in flake form, 15 g. lead naphthenate (24% solution) and 120 ml. of benzene to a reflux system and heating to about 115 C. and refluxing at that temperature for 6 hours. During this time water and benzene are distilled oif. During reflux 100 ml. of benzene are added. A total of 298 ml. of water was distilled over. The resin was found to be a benzylic ether type of phenolic resin (see US. Pat. 3,409,579). Fifty parts by weight of the resin is then blended with 30 parts furfuryl alcohol and 20 parts of aromatic solvent having a boiling range of 315 to 350 F.

The polyisocyanate is a commercially available product, which is a mixture of diand triphenylmethane, diand tri-isocyanate (e.g., Mondur MR from Mobay or NCO 120 from Kaiser Chemical). The phenolic and polyisocyanate components are used in a 1:1 weight ratio.

The aluminum oxide grains are placed in a tumbling barrel mixer along with the furfural alcohol and mixed until the grains are evenly wetted. The powdered novolak resin and the cryolite are mixed to form the bond composition which is slowly added to the tumbling wet abrasive grains over a period of three minutes and mixed until the powdered resin is evenly distributed. A portion of the mix is removed and the mixing ofboth portions continued. A sample of the removed portion revealed that it agglomerated upon standing for minutes without agitation. The control test wheels are made from the removed, control mix portion and the remaining portion processed as follows.

Lime, which acts as additional catalytic agent along with the hexa for the encapsulant system, is added in an amount of 0.5% by weight based on the weight of the remaining aggregate mix and mixing continued for 2 minutes to evenly distribute the lime. A sample of the mix at this point agglomerated after standing for to minutes without agitation. While continuing the mixing of the remaining portion of the mix, 1.0% by weight based on the weight of the mix of the encapsulant system is sprayed onto the tumbling mix and mixing continued for an additional 2 minutes. The mixing is stopped and the resulting encapsulated mix is free-flowing and non-tacky with no tendency to agglomerate upon standing for 24 hours.

The comparative testing of the control mix and the encapsulated mix are each screened through a #8 mesh screen and used to make 7 cut-01f wheels 16 inches in diameter, inch thick with a 1 inch arbor hole. Both sets of wheels are formed by cold pressing at 2500 psi. and baking for 40 hours at a maximum of 360 F. The cured wheels are each used to make 100 cuts of 1 inch round cold rolled steel bar stock and each spun to destruction. The following figures represent the average of all 7 values in each group of wheels.

Breakin spee Wheel M/W (r.p.m.)

Made from encapsulated mix 8. 46 4, 933 Made from control mix 7. 73 5, 450

EXAMPLE 2 Aggregate mix: Parts by weight Aluminum oxide (24 grit) 77 Furfuryl alcohol 2 Novolak resin (9.0% by weight hexa) 13 Cryolite 7 Iron pyrites 1 Encapsulant system:

Benzylic ether phenolic resin 50 'Polyisocyanate 50 The procedure of Example 1 is used with the exception that the test wheels are 20 inches in diameter by Vs inch and 10 cuts of 1 inch round stock are made with each wheel.

The results are as follows:

Breaking s ee Wheel M/W (r.p m.)

Made from encapsulated mix 4.54 4, 500 Made from control mix 4.54 4,100

EXAMPLE 3 In this example the materials and procedures are the same as in Example 1 except the lime is not added to the mix encapsulated and the mixing continued for 3 minutes after addition of the encapsulant system. The results obtained with the test wheels are essentially the same as in Example 1.

The following Examples 4-8 are the same as Example 1 in all respects except the following hydroxyl functional polymers are used with the polyisocyanate in the encapsulant system. The test wheels in the following give comparative test results of the control mix wheels and the encapsulated mix wheels similar to the comparative test results exhibited in Example 1.

EXAMPLE 4 The hydroxyl functional polymer used in this example is a novolak prepared by heating 73 parts by weight of 37% aqueous solution of formaldehyde, 100 parts of phenol, 0.2 part H 80 to boiling and refluxing at a pH below 1.2 for 2 hours. Ammonia added and boil-off conducted at 150 C. Vacuum is applied for steam distillation to a melting point of about 235 F. The mixture is cooled and 100 parts by Weight of the resulting resin cut in 100 parts cyclohexanone. The resulting solution is then mixed with the polyisocyanate for use as the encapsulant system.

EXAMPLE 5 The hydroxyl functional polymer of this example is an oil soluble type phenolic resin prepared by slowly heating 108 parts by weight 37 aqeuous formaldehyde, 100 parts p-tertiary butyl phenol and 27 parts 25% aqueous solution NaOH to the boiling point and refluxing for 12 minutes. Vacuum is applied to cool to C. and a mixture of 40 parts xylene and 60 parts 10% HCl is added, mixed and allowed to separate. The resin is washed free of salts and dehydrated under vacuum. Sixty parts of the resin are then mixed with 40 parts of 50/50 mixture of toluene and butyl Cellosolve acetate and mixed with the polyisocyanate for use as the encapsulant system.

EXAMPLE 6 The hydroxyl functional polymer of this example is a novolak/resole resin free of water prepared by heating 100 parts by weight phenol, parts 37% aqueous formaldehyde and 0.25 part concentrated H 50 to boiling and refluxing for 15 minutes. After cooling to 70 C. and adding 6 parts barium hydrate pre-slurried in 12 parts water, 135 parts 37% aqueous formaldehyde is slowly added and refiuxed for 15 minutes. The mixture is cooled and 13 parts 10% H 50 added and the mixture stripped under vacuum. Sixty parts are mixed with 40 parts butyl Cellosolve acetate and mixed with the polyisocyanate for use as the encapsulant system.

EXAMPLE 7 Tha hydroxyl functional polymer of this example is an epoxy resin prepared refluxing 100 parts by weight phenol, 1 part oxalic acid pre-dissolved in 1.5 parts water and 65 parts 37% aqueous formaldehyde for 2 hours. The reactor is converted to open boil-off and 150 parts of an epoxy novolak which is preheated to 100 C. is added and the mixture boiled to a temperature of 120 C. then cooled and cut with an equal weight of cyclohexanone for use as the encapsulant system when mixed with the polyisocyanate. The epoxy novola used in this example is a conventional commercially available novolak/epoxy resin of the type described in US. Pat. 2,658,885.

EXAMPLE 8 The hydroxyl functional polymer of this example is a polyester phenolic resin prepared by heating 636 parts by weight diethylene glycol, 888 parts phthalic anhydride and "0.7 part dibutyltinoxide to 340 F. and holding the mixture at that temperature for 30 minutes. Pentaerythritol in the amount of 138 parts is added and the mixture heated to 420-450 F. and held there for 7 hours under CO blanket. To 290 parts of the resulting resin 320 parts of the benzylic ether phenolic resin of Example 1 and 60 parts toluene are added and heated to 270 F. for 1 hour with azeotropic water removal followed by vacuum stripping of the toluene before cooling. Sixty parts of the resulting polyester phenolic resin is cut with 40 parts butyl Cellosolve acetate then mixed with the polyisocyanate for use as the encapsulant system.

EXAMPLE 9 This example illustrates the use of a different aggregate binder, an alkyd resin, which does not contain a catalyst for the encapsulant system. The catalyst is added separately to the mix before encapsulation.

Aggregate mix: Parts by weight Aluminum oxide (30 grit) 78 Cellosolve 2 Alkyd resin Lime 0.5

Encapsulant system:

Benzylic ether phenolic resin 50 Polysiocyanate 50 The alkyd resin is prepared by slowly heating 48 parts by weight glycerin and 100 parts phthalic anhydride in an open reactor to 350 F. and holding at that temperature for 14 hours to a melting range of 175 F. to 185 F. The resin is cooled and ground to a powder.

The aluminum oxide grains are placed in a tumbling barrel along with the Cellosolve and mixed until the grains are evenly wetted. The powdered alkyd resin is slowly added to the tumbling wet abrasive grains over a period of three minutes and mixed until the resin is evenly distributed. A sample of the mix this point agglomerated upon standing for 24 hours without agitation. The second is divided into three equal parts and the mixing of all three parts continued. Spraying part one of the mix with 1.0% by weight based on the weight of the mix of the encapsulant system produces an even more tacky mass which agglomerates upon standing for 10 minutes without agitation. The encapsulant system fails to cure due to the lack of a basic catalyst. Parts two and three of the mix are each dusted with 0.5% by weight based on the weight of mix of lime and tumbled for three minutes to assure even distribution of the lime. A sample of the mixes at this point agglomerates within 30 minutes when not agitated. Part two of the mix is used as is to form the control set of grinding wheels as in Example 1. Part three of the mix is sprayed with 1.0% by weight based on the weight of the mix of the encapsulant system and tumbled for three minutes. The resulting encapsulated mix is freeflowing and non-tacky with no tendency to agglomerate upon stanling for 24 hours without agitation. The second set of test wheels are formed as in Example 1.

While the durability of the alkyd bonded wheels is considerably lower than phenolic bonded wheels, the com parative durabilities of the control mix wheels and the encapsulated mix wheels have a relationship to one another similar to the comparative durabilities illustrated in Example 1. The comparative strengths are likewise similarly related.

We claim:

1. An abrasive mix containing encapsulated abrasive grains each grain having a resin binder selected from the group consisting of novolak and alkyd resins distributed on the surface thereof, which abrasive grain and accompanying binder is encapsulated in a shell of cross-linked thermoset polymer formed by the reaction of (a) a hydroxyl functional polymer selected from the group consisting of water-free phenol aldehyde resins, polyester resins containing phenolic hydroxy groups in a ratio of phenolic hydroxy groups to aliphatic hydroxy groups of at least 3 to 1, epoxy resins having a ratio of phenolic hydroxy groups to aliphatic hydroxy groups of at least 3 to l, and mixtures thereof, and (b) a polyisocyanate.

2. An abrasive mix according to claim 1 wherein the hydroxyl functional polymer is a benzylic ether phenolic resin.

3. A process for forming an abrasive mix, which comprises:

distributing a binder selected from the group consisting of novolak and alkyd resins and a catalytic agent selected from the group consisting of lime and hexamethyltetramine on the surface abrasive aggregate grains;

while mixing the aggregate having the binder and catalytic agent thereon, adding an encapsulant system comprising (a) a hydroxyl functional polymer selected from the group consisting of water-free phenol-aldehyde resins, polyester resins containing phenolic hydroxy groups in a ratio of phenolic hydroxy groups to aliphatic hydroxy groups of at least 3 to 1, epoxy resins having a ratio of phenolic hydroxy groups to aliphatic hydroxy groups of at least 3 to 1, and mixtures thereof, and (b) a polyisocyanate and continuing the mixing to evenly distribute the encapsulant system.

4. A process according to claim 3 wherein the waterfree phenol-aldehyde resin is selected from the group consisting of a benzylic ether phenolic resin, a novolak, and a resole.

5. A process according to claim 3 wherein the hydroxyl functional polymer is a benzylic ether phenolic resin.

6. A process according to claim 5 wherein the binder is a novolak resin and wherein the catalytic agent is hexamethylenetetramine.

7. A process for forming an abrasive article which comprises forming the abrasive mix of claim 1 into a 13 green abrasive article and heat-curing said green abrasive article to form the final abrasive article.

8. A process for forming an abrasive article which comprises forming the abrasive mix of claim 2 into a green abrasive article and heat-curing said green abrasive article to form the final abrasive article.

9. An abrasive article comprising a cured abrasive mix containing, before being cured, encapsulated abrasive grains having a binder selected from the group consisting of novolak and alkyd resins distributed on the surface of said abrasive grains, said grains and accompanying binder being encapsulated in a shell of cross-linked thermoset polymer formed by the reaction of (a) hydroxyl functional polymer selected from the group consisting of water-free phenol-aldehyde resins, polyester resins containing phenolic hydroxy groups in a ratio of phenolic hydroxy groups to aliphatic hydroxy groups of at least 3 to 1; epoxy resins having a ratio of phenolic hydroxy groups to aliphatic hydroxy groups of at least 3 to 1, and mixtures thereof; and (b) a polyisocyanate.

10. The abrasive article of claim 9 wherein the hydroxyl functional polymer is a benzylic ether phenolic resin.

References Cited UNITED STATES PATENTS 3,402,034 9/1968 Schnabel 51-295 3,255,500 6/ 1966 Engel et a1. 260-304 3,409,579 11/ 1968 Robins 260-304 3,406,020 10/1968 DAlessandro 51-298 2,959,474 11/ 1960 Daniels 51-298 3,041,156 6/1962 Rowse 51-298 3,225,495 12/ 1965 De Vries 51-298 3,489,541 1/ 1970 Steinberg 51-295 3,525,600 8/1970 Yoshikawa. et a1 51-295 2,878,111 3/1959 Daniels 51-298 DONALD J. ARNOLD, Primary Examiner US. Cl. X.R. 51-298 

